Nitrides such as silicon nitride and aluminum nitride, and oxynitrides such as silicon oxynitride, aluminum oxynitride and Sialon are reputed for their excellent mechanical and thermal properties. Silicon nitride has high resistance to heat and thermal shock. Having high strength both at ambient and elevated temperatures, silicon nitride holds much promise as one of the high-strength heat-resistant materials or high-precision wear-resistant materials that can be used to manufacture heat engines (e.g., diesel engines and gas turbines) that are lighter in weight and adapted to operation with higher efficiency while withstanding temperatures higher than those permitted by the conventional products. Aluminum nitride not only has high heat conductivity; it also exhibits excellent electrical properties such as high insulation resistance, breakdown voltage and dielectric constant, as well good mechanical properties such as high strength. Having these features, aluminum nitride has drawn researchers' attention as a material suitable for making high thermal conductive substrates and packaging.
The properties of the nitrides illustrated above and oxynitrides depend on the properties of the powder from which they are produced, so it is strongly desired to develop fine powders that have good sinterability and which can be obtained as high-purity and homogeneous products.
Nitride and oxynitride powders can be synthesized by direct nitridation of metals, reductive nitridation of oxides, pyrolysis of organometallic compounds containing nitrogenous components such as imides, or by gas-phase reaction with chlorides or some other suitable compounds. Among these methods, reductive nitridation of oxides is best suited to the purpose of preparing inexpensive but high-quality fine powders on an industrial scale.
In the reductive nitridation of oxides, it is common practice to perform a nitridation reaction on a powder mixture of silicon oxide or alumina and carbon in stacked trays by allowing a reactive gas such as nitrogen to pass through the trays. For successful production of the high-quality powders of nitrides or oxynitrides on an industrial scale, it is important that the reactive gas such as nitrogen be permitted into the individual trays in an efficient and uniform way and that the supplied gas be smoothly discharged from the trays after having participated in the desired reaction.
In conventional furnaces employed in the reductive nitridation of oxides, trays each having cutouts either in the top of two opposite side walls or of the four side walls are stacked on a base plate, and a reactive gas is introduced into the furnace at one end and discharged therefrom at the other end. In this method which is generally referred to as a "stacked tray" method, most of the reactive gas permitted into the furnace flows outside the walls of the stacked trays, and only part of it will flow into the trays, and in order to ensure that the reaction will take place to a satisfactory extent, it has been necessary to supply the reactive gas in an amount much greater than is theoretically required. As a further problem, it is difficult to allow a constant volume of reactive gas to be fed uniformly into each of the stacked trays, and this has caused variations in the quality of the reaction product from tray to tray.